STUDY OF ULTRA-HIGH PERFORMANCE CONCRETE UNDER HIGH RATE OF LOADING

Abstract

Concrete structures are often subjected to accidentally or deliberately generated impact and blast loading conditions. These extreme loading conditions describe the mechanics of deformation significantly distinct from that of the conventional loading. The magnitude of the induced stresses within the concrete is influenced by the rate of loading and the confining pressure. Therefore, it is important to study the response of cementitious material under high pressure, high strain rate and large deformation loading scenarios in order to design concrete structures properly for all spectrum of loading conditions. In the present study, comprehensive experimental, numerical, and analytical investigations were conducted on various cementitious concrete, focusing on both material and structural-level responses under different loading conditions. The experimental work involved dynamic material characterization of different concrete materials and ballistic experimentation testing of concrete targets subjected to rigid projectiles. Dynamic material characterization of concrete was studied by performing the uniaxial compression tests, tri-axial tests and the dynamic compression tests. Tri-axial compression experiments were conducted using a Hoek Cell, which can accommodate concrete specimens of diameter of 45 mm and height, 90 mm. Axisymmetric confining pressure in the Hoek Cell was generated manually through a pressure supply. The confining pressure in the tri-axial compression tests ranged up to 40 MPa. On the other hand, dynamic compression loading tests have been carried out using the indigenously developed and calibrated Split Hopkinson Pressure Bar (SHPB) test setup at various strain rates ranging from 90 to 360 s-1. The obtained results have also been investigated and compared in terms of failure mechanism, dynamic stress-stain curve, dynamic compressive strength (DCS), dynamic increase factor (DIF), maximum failure strain and toughness with respect to different strain rate loading. The ballistic experimentation has been carried out to study the structural behaviour of ultra-high performance concrete (UHPC) targets. The square targets of aerial dimensions 400 mm x 400 mm and thickness 55 mm were subjected to normal incidence by 450 gm ogive nosed (3 CRH) hardened long rod steel projectile of diameter 18 mm and length 245 mm. Additionally, plain and steel fiber normal strength concrete (NSC) targets of thicknesses 55 and 80 mm were also studied experimentally with respect to their ballistic performance for comparative analysis. The reinforced concrete (RC) targets of thickness 80 mm with six different reinforcement configurations were also studied under ballistic impact loading. The perforation tests were carried out at low impact velocities in the range, 70 – 195 m/s. Almost, all the slabs were subjected to impact under normal incidence and the projectile velocities were measured by using ii high-speed camera. The obtained results have been analyzed and compared with respect to extent and vicinity of surface damage, ballistic limit velocity (BLV) and ballistic perforation resistance etc. The experimentally obtained results were also validated through analytical study. Explanation regarding the behavior of different concrete targets against long rod projectile impact loading has been well addressed through the basic laws and relevant theories associated with ballistic impact loading in order to provide guidance for subsequent research in the field of protective structures design. The numerical investigation included the calibration of dynamic constitutive models for various concrete materials studied and the development of Finite Element Model (FEM) to simulate the response of concrete targets under different loading conditions using Abaqus-Explicit solver. The Holmquist-Johnson-Cook (HJC) model has been used for simulating the behavior of concrete under high strain rate, high pressure, and large deformation conditions owing to its simplicity and accuracy. In the current study, the set of dominant parameters of HJC model has been calibrated for normal strength concrete (NSC), ultra-high performance concrete (UHPC) and short steel fibre ultra-high performance concrete (SS-UHPC) through the data obtained from dynamic material characterization tests. Numerical simulations were carried out on a single element in order to study its response under different loading conditions for reproducing the material behaviour of concrete. Subsequently, the FE model incorporating the calibrated parameters was developed for numerical simulations to reproduce the ballistic experimental outcomes. The results indicated a close agreement between the finite element simulations and ballistic experimentation, and allowed a detailed examination of the dynamic response of concrete as a material and structural perspective. A parametric study using the developed FE model was conducted to gain insight into the influence of various material parameters of the HJC model under ballistic impact loading. The investigation focused on the influence of specific material properties, including compressive strength, toughness, normalized cohesive strength, pressure hardening parameter, strain rate sensitivity, tensile strength, and normalized maximum strength etc. The current study provided essential information for suitably estimating and improving the material and structural behaviour of concrete under dynamic applications.